Method of making a distributed optical fiber sensor having enhanced rayleigh scattering and enhanced temperature stability, and monitoring systems employing same

ABSTRACT

A method of making an optical fiber sensor device for distributed sensing includes generating a laser beam comprising a plurality of ultrafast pulses, and focusing the laser beam into a core of an optical fiber to form a nanograting structure within the core, wherein the nanograting structure includes a plurality of spaced nanograting elements each extending substantially parallel to a longitudinal axis of optical fiber. Also, an optical fiber sensor device for distributed sensing includes an optical fiber having a longitudinal axis, a core, and a nanograting structure within the core, wherein the nanograting structure includes a plurality of spaced nanograting elements each extending substantially parallel to the longitudinal axis of the optical fiber. Also, a distributed sensing method and system and an energy production system that employs such an optical fiber sensor device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) from U.S.provisional patent application No. 62/552,619, entitled “Method ofMaking a Distributed Optical Fiber Sensor Having Enhanced RayleighScattering and Enhanced Temperature Stability, and Monitoring SystemsEmploying Same” and filed on Aug. 31, 2017, the contents of which areincorporated herein by reference.

GOVERNMENT CONTRACT

This invention was made with government support under grant #CMMI-1300273 awarded by the National Science Foundation (NSF) and grant# DE-FE0028992 awarded by the Department of Energy (DOE). The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to optical fiber sensor devices for makingdistributed sensing measurements (e.g., of temperature and/or chemicalcomposition) in, for example, reactor systems, such as, withoutlimitation, solid oxide fuel cell (SOFC) based power plant systems, and,in particular, to a method of making a distributed sensing optical fibersensor device using ultrafast laser irradiation that has an enhancedRayleigh scattering profile and enhanced temperature stability, and toimproved monitoring systems that employ such enhanced distributedsensing optical fiber sensor devices.

2. Description of the Related Art

Solid oxide fuel cell (SOFC) technology is a promising and versatileenergy conversion scheme. SOFCs are used in a wide variety ofapplications, ranging from clean automobiles to distributed electricpower generation systems. SOFCs in stand-alone or hybrid generationconfigurations can utilize a wide array of gaseous fuels, from hydrogento biogas, to achieve high energy conversation efficiencies and lowemissions.

Modern SOFC systems exhibit high internal reaction temperatures, and arecapable of internal gas-reforming to provide fuel flexibility andincreased versatility. As a result, during normal use, a typical SOFCassembly will experience significant thermal stresses resulting fromtemperature variations that occur within the fuel assembly. Inparticular, significant temperature variations are produced within anSOFC assembly as a result of a number of different processes, includingconvection as gases with varying thermal conductivities flow through theSOFC assembly, conduction of heat through the supporting structure ofthe SOFC assembly, and heat generated by the distributed internalreforming or oxidation reactions that produce the electrical output.Over time, these temperature variations and the resulting thermalstresses cause material and/or structural degradation of the SOFCassembly. Such degradation negatively impacts the long-term stability ofan SOFC system and ultimately negatively impacts the profitability oflarge-scale SOFC system deployment.

Thus, the ability to measure, understand and engineer the temperaturedistribution inside of an SOFC assembly is essential to improving thelongevity of SOFC systems.

Currently, numerical simulations are used to estimate temperaturedistributions in SOFC systems because experimental measurements duringoperation have been extremely challenging. Thermocouple devices havebeen used by some researchers to perform single-point measurements. Asmany as thirty six thermocouples have been inserted in a fuel cell stackto perform multi-point measurements. However, each thermocouple requirestwo electrical wires for each point-measurement, and each wireintroduces additional heat-losses which skews the accuracy ofmeasurements. It is also physically impossible to place thermocouples inextremely close proximity to one another in order to produce highspatial-resolution measurements. Large numbers of wires in the SOFCassembly could also impede fuel gas-flow, which may further alter thetemperature profile during measurement.

Distributed fiber optic sensing is a potentially powerful technique tomeasure the spatial temperature profile of an operating SOFC system.Being well-suited for harsh environment sensing applications, fiberoptic sensors have been widely used for high temperature measurements.Distributed sensing schemes such as Rayleigh-scattering OpticalFrequency Domain Reflectometry (OFDR) can perform distributedtemperature sensing using unmodified single-mode optical fiber toachieve <1-cm spatial resolution. However, one of the key challenges ofdistributed sensing using Rayleigh backscattering is the weakRayleigh-backscattering intensity exhibited by conventional opticalfibers. Such weak Rayleigh backscattering is hardly a surprise, giventhat telecommunications optical fibers are designed for low-losses,including low Rayleigh-scattering losses.

To address this challenge, several approaches have been attempted toincrease Rayleigh scattering in single-mode fibers. The resultingenhanced Rayleigh scattering profiles result in larger scatteringsignals at the detector(s) and better spectral correlation qualitybetween the measured high-temperature Rayleigh profile and the referenceroom-temperature Rayleigh profile. This in turn improves the fidelity ofthe distributed measurement as well as the useful range of temperaturesover which the sensor-fiber can operate effectively. However, eventhough Rayleigh scattering enhancement may be used to effectively extendthe operational temperature and longevity of stable measurements, incurrent enhancement approaches environmental effects will eventuallyovercome the stability of the measurement during extended periods ofhigh-temperature operation (>700° C.); with the maximum stableoperational temperature depending on the surrounding chemicalenvironment. At the highest operating temperatures, the same fiber corestructural non-uniformities that give rise to the intrinsic Rayleighscattering undergo permanent changes that compromise the sensitivity andreliability of distributed measurements.

There is thus room for improvement in distributed fiber optic sensingfor applications that require high temperature stability, particularlyin harsh chemical environments.

SUMMARY OF THE INVENTION

In one embodiment, a method of making an optical fiber sensor devicestructured for distributed sensing is provided. The method includesgenerating a laser beam comprising a plurality of ultrafast pulses, andfocusing the laser beam into a core of an optical fiber to form ananograting structure within the core, wherein the nanograting structureincludes a plurality of spaced nanograting elements each extendingsubstantially parallel to a longitudinal axis of optical fiber.

In another embodiment, an optical fiber sensor device structured fordistributed sensing is provided that includes an optical fiber having alongitudinal axis, a core, and a nanograting structure within the core,wherein the nanograting structure includes a plurality of spacednanograting elements each extending substantially parallel to thelongitudinal axis of the optical fiber.

In still another embodiment, a distributed sensing method is providedthat includes transmitting an interrogating light through an opticalfiber sensor device as just described above, receiving a Rayleighscattering profile from the optical fiber sensor device in response tothe interrogating light, and determining a plurality of spatiallyresolved measurements based on the Rayleigh scattering profile.

In yet another embodiment, a distributed sensing system is provided thatincludes a light source structured to generate an interrogating light,an optical fiber sensor device as just described above structured toreceive the interrogating light, and an optical frequency domainreflectometry sensing system structured and configured to receive aRayleigh scattering profile from the optical fiber sensor device inresponse to the interrogating light, and determine a plurality ofspatially resolved measurements based on the Rayleigh scatteringprofile.

In still a further embodiment, an energy production system is providedthat includes a reactor assembly, an optical fiber sensor device as justdescribed above provided within the reactor assembly, a light sourcestructured to generate an interrogating light, wherein the optical fibersensor device is structured to receive the interrogating light, and anoptical frequency domain reflectometry sensing system structured andconfigured to receive a Rayleigh scattering profile from the opticalfiber sensor device in response to the interrogating light, anddetermine a plurality of spatially resolved measurements based on theRayleigh scattering profile.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system for manufacturing an opticalfiber sensor device according to an exemplary embodiment of thedisclosed concept;

FIG. 2 is an enlarged view of a portion of the optical fiber sensordevice manufactured using the system of FIG. 1 and the methodologydescribed in detail herein;

FIGS. 3A, 3B and 3C show the enhanced Rayleigh backscattering profilesof three exemplary optical fiber sensor devices made using the ultrafastlaser direct writing scheme of the disclosed concept;

FIG. 4 is a schematic diagram of an SOFC system having distributedsensing capabilities according to one particular exemplary embodiment ofthe disclosed concept; and

FIG. 5 is a schematic of a cross-section taken a long the z directionshown in FIG. 2 according to an exemplary embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As used herein, the singular form of “a”, “an”, and “the” include pluralreferences unless the context clearly dictates otherwise.

As used herein, the statement that two or more parts or components are“coupled” shall mean that the parts are joined or operate togethereither directly or indirectly, i.e., through one or more intermediateparts or components, so long as a link occurs.

As used herein, “directly coupled” means that two elements are directlyin contact with each other.

As used herein, the term “number” shall mean one or an integer greaterthan one (i.e., a plurality).

As used herein, the term “ultrafast pulse” shall mean an electromagneticpulse whose time duration is on the order of one nanosecond or less.

As used herein, the term “ultrafast laser system” shall mean a lasersystem that generates and emits laser pulses that are ultrafast pulses.

As used herein, the term “femtosecond ultrafast pulse” shall mean anultrafast pulse whose time duration is on the order of 500 femtosecondsor less.

As used herein, the term “femtosecond ultrafast laser system” shall meanan ultrafast laser system that generates and emits laser pulses that arefemtosecond ultrafast pulses.

As used herein, the term “substantially parallel” shall mean exactlyparallel or within ±10° of exactly parallel.

As used herein, the term “substantially perpendicular” shall meanexactly perpendicular or within ±10° of exactly perpendicular.

Directional phrases used herein, such as, for example and withoutlimitation, top, bottom, left, right, upper, lower, front, back, andderivatives thereof, relate to the orientation of the elements shown inthe drawings and are not limiting upon the claims unless expresslyrecited therein.

The disclosed concept will now be described, for purposes ofexplanation, in connection with numerous specific details in order toprovide a thorough understanding of the disclosed concept. It will beevident, however, that the disclosed concept can be practiced withoutthese specific details without departing from the spirit and scope ofthis innovation.

As described in greater detail herein, the disclosed concept usesultrafast (e.g., femtosecond) laser irradiation with sub-μJ laser pulses(e.g., 300-nJ laser pulses) to produce optical fiber sensor devices thathave enhanced Rayleigh scattering profiles and that are stable at high(e.g., 400° C. and higher) temperatures. The resultant enhanced opticalfiber sensor devices can, in the exemplary embodiment, performdistributed temperature sensing with very small (e.g., 1 mm) spatialresolution at very high temperatures (e.g. 800° C.) in highly reactivefuel gas (e.g., hydrogen) streams. Using this powerful sensing tool,distributed temperature measurements may be performed in harshenvironments such as, without limitation, an operating SOFC system. Inparticular, in certain exemplary embodiments, information gathered bythe optical fiber sensor devices of the disclosed concept can becompared with simulation results to aid in SOFC system design, or usedto perform in-vivo monitoring of an active cell, and ultimately improvethe operational efficiency and longevity of SOFC systems (or otherenergy production systems).

FIG. 1 is a schematic diagram of a system 2 for manufacturing an opticalfiber sensor device 4 according to an exemplary embodiment of thedisclosed concept. FIG. 2 is an enlarged view of a portion of opticalfiber sensor device 4 manufactured using the system 2 and themethodology described in detail herein. As described in greater detailherein, and as seen in FIG. 2, optical fiber sensor device 4 includes ananograting structure 6 (described in detail below) that is provided inthe core 8 of an optical fiber 10 forming part of optical fiber sensordevice 4. In the exemplary embodiment, optical fiber 10 is asilica-based optical fiber. However, it will be appreciated that othertypes of optical fibers, such as single-crystal optical fibers like asapphire-based optical fibers other single-crystal optical fibers, mayalso be employed within the scope of the disclosed concept.

Nanograting structure 6 is the mechanism that enables optical fibersensor device 4 of the disclosed concept to provide enhanced Rayleighscattering profiles and to be stable at high temperatures. As seen inFIG. 2, nanograting structure 6 includes a plurality of nanogratingelements 12, labeled 12 a-12 c. Each nanograting element 12 comprises aportion of core 8 that has been caused to have a different, lowerrefractive index than the surrounding portions of core 8 as a result ofthe laser irradiation (and the damage caused thereby) that is describedherein. For example, each nanograting element 12 may have a refractiveindex that ranges from the core index down to 90% of the refractiveindex of core 8. Other refractive index contrasts may also beachievable. Furthermore, each nanograting element 12 is generally planarand has a length that extends in the z direction of FIG. 2 along (i.e.,substantially parallel to) the longitudinal axis of optical fiber 10. Inaddition, each nanograting element 12 has a width that extends in the xdirection of FIG. 2 (substantially perpendicular to the longitudinalaxis) and a height that extends in the y direction of FIG. 2 (alsosubstantially perpendicular to the longitudinal axis). Also, as seen inFIG. 2, regions of substantially parallel reduced index or “nanogratingelements 12” are spaced from one another in the x direction asdetermined by the ultrafast laser wavelength.

In one particular, non-limiting exemplary embodiment, the spacingbetween each pair of immediately adjacent nanograting elements 12 issignificantly smaller than (in one particular embodiment less than orequal to one-fifth of) the wavelength of the interrogating light that isto be used to interrogate optical fiber sensor device 4 when in use. Inanother particular, non-limiting exemplary embodiment, the spacingbetween each pair of immediately adjacent nanograting elements 12 isless than or equal to one-tenth of the wavelength of the interrogatinglight that is to be used to interrogate optical fiber sensor device 4when in use. These embodiments are produced via alteration of theultrafast laser wavelength. These embodiments will reduce the likelihoodthat interrogation wavelength dependent loss features will be presentduring use.

Furthermore, in another particular, non-limiting exemplary embodiment,the spacing between each immediately adjacent pair of nanogratingelements 12 is 750 nm or less, or, alternatively, 500 nm or less or 250nm or less. In any case, the height and width of nanograting elements 12depends on the laser energy deposited into the focal volume, which isalso determined by the laser scanning speed.

Referring again to FIG. 1, the method by which nanograting structure 6is formed in core 8 of optical fiber 10 using system 2 will now bedescribed. As seen in FIG. 1, system 2 includes an ultrafast lasersystem 14 that is structured and configured to generate a laser beam 16comprising a plurality of ultrafast pulses, and a pair of cylindricallenses 18 that are structured to spatially shape and focus laser beam 16into core 8 of optical fiber 10 (possibly through a focusing objective).System 2 also includes a computer controlled, air-bearing motion stage20 that is structured to hold and support optical fiber 10 and moveoptical fiber 10 longitudinally relative to cylindrical lenses 18 andlaser beam 16 in the direction shown by the arrow in FIG. 1. Inoperation, laser beam 16 is focused into core 8 as just described whileoptical fiber 10 is moved longitudinally relative to laser beam 16 bymotion stage 20. This laser irradiation, consisting of a continuouspulse-train of ultrafast pulses, causes nanograting 6 to be producedwithin core 8 as shown and described in connection with FIG. 2. Morespecifically, as a result of the shaping and focusing of laser beam 16by cylindrical lenses 18, the laser irradiation will have an intensityprofile that causes nanograting elements 12 to be formed in core 8 ofoptical fiber 10, thereby forming optical fiber sensor device 4.Referring to FIG. 2, S is the scanning/translation direction of laserbeam 16 during the method just described, E is the direction of theelectrical field, and k is the nanograting orientation direction and thedirection of writing light propagation of laser beam 16 during themethod just described.

While in the embodiment just described optical fiber 10 is movedrelative to a stationary laser beam 16, it will be appreciated thatinstead the laser beam may be moved relative to a stationary opticalfiber 10 without departing from the scoped the disclosed concept.

In addition, in the embodiment just described, optical fiber 10 is acylindrical fiber having a cylindrical core 8 and cylindrical claddingsurrounding core 8. Alternatively, optical fiber 10 may be a so calledD-shaped fiber or a so-called solid-core photonic bandgap fiber.

Moreover, in one non-limiting exemplary embodiment, ultrafast lasersystem 14 is a Coherent femtosecond ultrafast laser system that consistsof a Coherent MIRA-D Ti:sapphire seed oscillator and a RegA 9000regenerative amplifier operated at 800 nm with a repetition rate of 250kHz. In this exemplary implementation, the pulse width of laser beam 16is 300-fs. Also in this exemplary embodiment, the output of cylindricallenses 18 is focused using an 80× microscope objective.

In addition, the exemplary system 2 as just described was used by thepresent inventors to create a number of exemplary optical fiber sensordevices 4 in 20 cm long sections of a silica based optical fiber 10 thatwas translated over 20 cm by motion stage 20 by varying the translationspeed of motion stage 20 from 0.1 mm/s to 1 mm/s. Specifically, scanningspeeds of 0.1 mm/s, 0.5 mm/s, and 1 mm/s were used to create threeexemplary optical fiber sensor devices 4. FIGS. 3A, 3B and 3C show theenhanced Rayleigh backscattering profiles of the three exemplary opticalfiber sensor devices 4 using the ultrafast laser direct writing schemeof the disclosed concept (FIG. 3A shows the 0.1 mm/s example, FIG. 3Bshows the 0.5 mm/s example, and FIG. 3C shows the 1 mm/s example). Ineach case, the on-target pulse energy was set at 300-nJ, which wasdetermined to be slightly above the threshold pulse energy required toenhance the Rayleigh backscattering in the fused-silica fiber. As seenin FIGS. 3a-3c , an increase in the Rayleigh backscattering amplitude of40-45 dB was obtained with laser irradiation for all of the chosenwriting speeds. The ultrafast laser irradiation did not yield Rayleighscattering enhancements when the laser writing speed exceeded 2 mm/s.The laser irradiation also introduced significant optical propagationlosses in the irradiated fiber-samples, which is characterized by theslope of the Rayleigh-enhanced region. At 300-nJ pulse energy, theaverage propagation losses in the irradiation sections are 0.41 dB/cm,0.30 dB/cm and 0.15 dB/cm, respectively. Therefore, in the exemplaryimplementation just described, a scanning speed of 1 mm/s was theoptimal processing condition that minimized the insertion loss of theirradiated sensor-segment.

According to a further aspect of the disclosed concept, optical fibersensor device 4 as described herein may be subjected to an annealingprocess in order to further increase the stability thereof. Inparticular, in the exemplary embodiment, optical fiber sensor device oris annealed in a tube furnace during a process wherein the temperatureof the furnace is ramped up from room temperature to a very hightemperature, such as 800° C., in air and held at that temperature for apredetermined period of time, such as 4 hours. Thereafter, the opticalfiber sensor device 4 is subjected to a reactive gas mixture, such as agas mixture including hydrogen (e.g. 10% hydrogen), at the hightemperature. In this embodiment, the high temperature annealing in thepresence of a reactive gas causes a plurality of voids or nanopores 52(e.g., spherical voids) to be formed in the core 8 of optical fiber 10as shown in FIG. 5, which is a schematic of a cross-section taken a longthe z direction shown in FIG. 2. In one embodiment, each of the voids 52has a diameter on the order of 50 nm or less. As seen in FIG. 5, in theexemplary embodiment, voids 52 are at least partially provided withineach of the nanograting elements 12 and are aligned anisotropicallyalong the location of the nanograting elements 12. In the presentembodiment, the annealing process causes the nanograting elements 12 tocoalesce and form voids 52. The presence of voids 52 results in enhancedstability of the scattering. Furthermore, the regions that include voids52 will demonstrate a larger index contrast as compared to the remainderof core 8. In one embodiment, the index contrast is on the order of 33%(i.e., the portion including voids 52 has an index of refraction that is33% less than the index of refraction of core 8).

Another feature of optical fiber sensor device 4 is the fact that thebackscattered amplitude will be different for the s and p polarizationsof the backscattered signal. In other words, the backscattered amplitudeof optical fiber sensor device 4 is polarization dependent. Suchpolarization dependence is caused by the anisotropy of nanogratingelements 12. This feature can potentially be used for extractingadditional value from nanograting elements 12, such as multi-parameteranalysis due to the birefringence of the structures.

As noted elsewhere herein, optical fiber sensor device 4 of thedisclosed concept may be used to perform distributed sensing (e.g.,distributed temperature sensing) in harsh, high temperature environmentssuch as those employed in various energy production processes frombiomass reactors, to SOFC systems to nuclear reactors, among others.FIG. 4 is thus a schematic diagram of an SOFC system 22 havingdistributed sensing capabilities according to one particular exemplaryembodiment of the disclosed concept which illustrates the usefulness ofoptical sensor device 4 as described herein in this regard.

Referring to FIG. 4, SOFC system 22 includes an SOFC assembly 24 thatincludes a first conductive interconnect 26, a second conductiveinterconnect 28, an anode layer 30, a cathode layer 32, and anelectrolyte layer 34. In operation, a fuel stream (such as H₂ gas) isprovided through SOFC assembly 24 under anode layer 30 in a firstdirection via passageways 36 provided in first conductive interconnect26, and an air stream is provided through SOFC assembly 24 over cathodelayer 32 in a second direction via passageways 38 provided in secondconductive interconnect 28 as shown in FIG. 4. The exemplary SOFCassembly 24 shown in FIG. 4 is a “counter-flow” configuration, meaningthe paths of the fuel stream and the air stream are in oppositedirections. It will be understood that this is meant to be exemplaryonly and that other types of SOFC assemblies are known and arecontemplated within the scope of the disclosed concept. In the cathodelayer 32, oxygen in the air stream is reduced into oxygen ions. Thoseions then diffuse through electrolyte layer 34 to anode layer 30, wherethey electrochemically oxidize the fuel in the fuel stream. In thisreaction, a water byproduct is given off as well as electrons. Thoseelectrons then flow through an external circuit to produce electricalcurrent.

As seen in FIG. 4, SOFC system 22 further includes a first optical fibersensor device 4 (labelled 4A) that is inserted through one of thepassageways 36 within a nickel tube 40 (labelled 40A) for purposes ofperforming distributed temperature sensing with spatial resolution asdescribed herein within SOFC assembly 24 between anode layer 30 andfirst conductive interconnect 26. Similarly, SOFC system 22 furtherincludes a second optical fiber sensor device 4 (labelled 4B) that isinserted through one of the passageways 38 within a nickel tube 40(labelled 40B) for purposes of performing distributed temperaturesensing with spatial resolution as described herein within SOFC assembly24 between cathode layer 32 and second conductive interconnect 28. Firstoptical fiber sensor device 4A and second optical fiber sensor device 4Bare both selectively coupled to an OFDR sensing system 42 through anoptical swatch 44. In the illustrated embodiment, OFDR sensing system 42includes a laser source 46, a circulator 48, and an optical frequencydomain reflectometer 50 that is structured and configured to makedistributed temperature sensing measurements based on the enhancedRayleigh backscatter profiles that are generated by first optical fibersensor device 4A and second optical fiber sensor device 4B (in responseto an integrating laser light) using known or hereafter developedRayleigh backscatter sensing techniques.

Accordingly, by employing ultrafast (e.g., femtosecond) laserirradiation with sub-μJ pulses, the Rayleigh scattering profile fromcommercially available silica fibers can be enhanced by more than 50-dBthrough nanograting formations in the fiber core. This increases theavailable measurable intensity at the optical detectors of an OFDRsystem, leading to significant improvements in both the Signal to noiseratio (SNR) and spectral shift quality of an OFDR-based measurement. Thenew Rayleigh backscatter features induced by the laser irradiationtechnique of the disclosed concept are stable at high temperatures,which enables

reliable temperature measurements in extreme environments. Thistechnique therefore represents a powerful new tool to potentially studya wide range of energy production processes from biomass reactors, tosolid oxide fuel cells, to monitoring in nuclear reactors. Using thedistributed sensing tool of the disclosed concept, reliable temperaturemeasurements can be achieved from room temperature to 800° C. or higher.The disclosed concept may thus be used to probe an operating SOFC'stemperature dependence on fuel stream inlet chemistry and fuelutilization that was previously inaccessible using known techniques. Thecapability for in-situ temperature monitoring with high spatialresolution within operational energy conversion devices such as solidoxide fuel cells represents a significant opportunity for processefficiency and long-term stability, which are two key metrics requiredfor enabling widespread deployment of SOFCs in the power generationsector. The system described herein may also be useful for measurementsin existing harsh-environment energy systems including combustionsystems, boilers, and gas turbines. Other spatially resolvedmeasurements, such as, without limitation, spatially resolved strainmeasurements or spatially resolved chemical composition measurements,are also possible within the scope of the disclosed concept.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word “comprising” or “including”does not exclude the presence of elements or steps other than thoselisted in a claim. In a device claim enumerating several means, severalof these means may be embodied by one and the same item of hardware. Theword “a” or “an” preceding an element does not exclude the presence of aplurality of such elements. In any device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain elements are recited in mutuallydifferent dependent claims does not indicate that these elements cannotbe used in combination.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, it is to be understood thatthe present invention contemplates that, to the extent possible, one ormore features of any embodiment can be combined with one or morefeatures of any other embodiment.

What is claimed is:
 1. A method of making an optical fiber sensor devicestructured for distributed sensing, comprising: generating a laser beamcomprising a plurality of ultrafast pulses; and focusing the laser beaminto a core of an optical fiber to form a nanograting structure withinthe core, wherein the nanograting structure includes a plurality ofspaced nanograting elements each extending substantially parallel to alongitudinal axis of optical fiber.
 2. The method according to claim 1,wherein the plurality of ultrafast pulses comprises a plurality offemtosecond ultrafast pulses.
 3. The method according to claim 2,wherein each of the femtosecond ultrafast pulses is a sub-μJ laserpulse.
 4. The method according to claim 1, further comprising causingthe laser beam and the optical fiber to be moved relative to one anotherduring focusing such that the nanograting structure is formed within alongitudinally extending portion of the core.
 5. The method according toclaim 4, wherein a position of the laser beam in a longitudinaldirection is stationary and wherein the optical fiber is caused to moverelative to the laser beam.
 6. The method according to claim 4, whereinthe optical fiber is stationary and wherein the laser beam is caused tomove relative to the optical fiber.
 7. The method according to claim 1,wherein a portion of the core surrounding the nanograting structure hasa first refractive index and wherein each nanograting element has asecond refractive index that is lower than the first refractive index.8. The method according to claim 7, wherein the second refractive indexis 99% to 90% of the first refractive index.
 9. The method according toclaim 1, wherein each nanograting element is generally planar and has alength that extends in a first direction that is substantially parallelto the longitudinal axis of the optical fiber, a width that extends in asecond direction that is substantially perpendicular to the longitudinalaxis of the optical fiber, and a height that extends in the thirddirection that is substantially perpendicular to the longitudinal axisof the optical fiber.
 10. The method according to claim 9, wherein thenanograting elements are spaced from one another in the seconddirection.
 11. The method according to claim 9, wherein the opticalfiber sensor device is designed for use with an interrogating lighthaving a wavelength, and wherein a spacing between each pair ofimmediately adjacent nanograting elements is less than or equal toone-fifth of the wavelength of the interrogating light.
 12. The methodaccording to claim 9, wherein the optical fiber sensor device isdesigned for use with an interrogating light having a wavelength, andwherein a spacing between each pair of immediately adjacent nanogratingelements is less than or equal to one-tenth of the wavelength of theinterrogating light.
 13. The method according to claim 9, wherein aspacing between each immediately adjacent pair of nanograting elementsis 750 nm or less.
 14. The method according to claim 1, wherein theoptical fiber is a single-crystal optical fiber.
 15. The methodaccording to claim 14, wherein the optical fiber is a sapphire opticalfiber.
 16. An optical fiber sensor device structured for distributedsensing, comprising: an optical fiber having a longitudinal axis, acore, and a nanograting structure within the core, wherein thenanograting structure includes a plurality of spaced nanogratingelements each extending substantially parallel to the longitudinal axisof the optical fiber.
 17. The optical fiber sensor device according toclaim 16, wherein a portion of the core surrounding the nanogratingstructure has a first refractive index and wherein each nanogratingelement has a second refractive index that is lower than the firstrefractive index.
 18. The optical fiber sensor device according to claim17, wherein the second refractive index is 99% to 90% of the firstrefractive index.
 19. The optical fiber sensor device according to claim16, wherein each nanograting element is generally planar and has alength that extends in a first direction that is substantially parallelto the longitudinal axis of the optical fiber, a width that extends in asecond direction that is substantially perpendicular to the longitudinalaxis of the optical fiber, and a height that extends in the thirddirection that is substantially perpendicular to the longitudinal axisof the optical fiber.
 20. The optical fiber sensor device according toclaim 19, wherein the nanograting elements are spaced from one anotherin the second direction.
 21. The optical fiber sensor device accordingto claim 20, wherein the optical fiber sensor device is designed for usewith an interrogating light having a wavelength, and wherein a spacingbetween each pair of immediately adjacent nanograting elements is lessthan or equal to one-fifth of the wavelength of the interrogating light.22. The optical fiber sensor device according to claim 20, wherein theoptical fiber sensor device is designed for use with an interrogatinglight having a wavelength, and wherein a spacing between each pair ofimmediately adjacent nanograting elements is less than or equal toone-tenth of the wavelength of the interrogating light.
 23. The opticalfiber sensor device according to claim 20, wherein a spacing betweeneach immediately adjacent pair of nanograting elements is 750 nm orless.
 24. The optical fiber sensor device according to claim 16, whereinthe optical fiber is a single-crystal optical fiber.
 25. The opticalfiber sensor device according to claim 24, wherein the optical fiber isa sapphire optical fiber.
 26. A distributed sensing method, comprising:transmitting an interrogating light through an optical fiber sensordevice according to claim 16; receiving a Rayleigh scattering profilefrom the optical fiber sensor device in response to the interrogatinglight; and determining a plurality of spatially resolved measurementsbased on the Rayleigh scattering profile.
 27. The distributed sensingmethod according to claim 26, wherein the plurality of spatiallyresolved measurements are a plurality of spatially resolved temperaturemeasurements along the optical fiber sensor device.
 28. The distributedsensing method according to claim 26, wherein the plurality of spatiallyresolved measurements are a plurality of spatially resolved strainmeasurements along the optical fiber sensor device.
 29. The distributedsensing method according to claim 26, wherein the plurality of spatiallyresolved measurements are a plurality of spatially resolved chemicalconcentration measurements along the optical fiber sensor device.
 30. Adistributed sensing system, comprising: a light source structured togenerate an interrogating light; an optical fiber sensor deviceaccording to claim 16 structured to receive the interrogating light; andan optical frequency domain reflectometry sensing system structured andconfigured to receive a Rayleigh scattering profile from the opticalfiber sensor device in response to the interrogating light, anddetermine a plurality of spatially resolved measurements based on theRayleigh scattering profile.
 31. The distributed sensing systemaccording to claim 30, wherein the plurality of spatially resolvedmeasurements are a plurality of spatially resolved temperaturemeasurements along the optical fiber sensor device.
 32. An energyproduction system, comprising: a reactor assembly; an optical fibersensor device according to claim 16 provided within the reactorassembly; a light source structured to generate an interrogating light,wherein the optical fiber sensor device is structured to receive theinterrogating light; and an optical frequency domain reflectometrysensing system structured and configured to receive a Rayleighscattering profile from the optical fiber sensor device in response tothe interrogating light, and determine a plurality of spatially resolvedmeasurements based on the Rayleigh scattering profile.
 33. The energyproduction system according to claim 32, wherein the plurality ofspatially resolved measurements are a plurality of spatially resolvedtemperature measurements along the optical fiber sensor device.
 34. Theenergy production system according to claim 28, wherein the reactorassembly is a solid oxide fuel cell assembly.
 35. The method accordingto claim 1, further comprising annealing the optical fiber sensor deviceafter the formation of the nanograting structure in the presence of areactive gas.
 36. The method according to claim 35, wherein the reactivegas is hydrogen.
 37. The method according to claim 35, wherein theannealing is performed at a temperature that is greater than or equal to700° C.
 38. The method according to claim 37, wherein the annealing isperformed at a temperature that is greater than or equal to 800° C. 39.The method according to claim 35, wherein the annealing causes aplurality of voids to be formed within the core.
 40. The methodaccording to claim 35, wherein the annealing causes a plurality of voidsto be formed at least partially within each of the nanograting elements.41. The method according to claim 40, wherein each of the voids isspherical.
 42. The optical fiber sensor device according to claim 16,wherein the core includes a plurality of voids formed by annealing theoptical fiber sensor device after the formation of the nanogratingstructure in the presence of a reactive gas.
 43. The optical fibersensor device according to claim 16, wherein a plurality of voids areincluded at least partially within each of the nanograting elements byannealing the optical fiber sensor device after the formation of thenanograting structure in the presence of a reactive gas.
 44. The opticalfiber sensor device according to claim 43, wherein each of the voids isspherical.
 45. The optical fiber sensor device according to claim 16,wherein the nanograting structure causes a backscattered amplitude ofbackscatter signals produced from the optical fiber sensor device to bepolarization dependent.